1. Field of the Invention
The present invention relates to an optical spectrum analyzer which uses spectroscope elements of the dispersion type such as a diffraction grating and prism and, more particularly, it relates to an optical spectrum analyzer capable of measuring, with high accuracy, the absolute value of an optical spectrum at every wavelength of light to be measured.
2. Description of the Related Art
In the case of the conventional optical spectrum. analyzer which measures an optical spectrum value at every wavelength .lambda. of light to be measured, the measured light entering through the inlet slit is introduced onto and diffracted by the diffraction grating. The light thus diffracted is received by the light receiving means through the outlet slit. An optical strength representing signal which is calculated from a photoelectric conversion signal corresponding to an angle .phi. of rotation of the diffraction grating and which is applied from the light receiving means is regarded as an optical spectrum value at every wavelength of the light to be measured.
As shown in FIG. 11, however, the polarization direction in the measured light is not certain but is optional to have an angle .alpha. relative to grooves of the diffraction grating 3. As shown in FIG. 12, the wavelength sensitivity characteristics P(.lambda.) of the grating in a direction parallel to its grooves is different from the wavelength sensitivity characteristics S(.lambda.) in a direction perpendicular to its grooves. Even when the measured light having the same wavelength components is subjected to spectrum analysis, therefore, the ratio of light (component P) polarized parallel to the grooves of the grating and light (component S) polarized perpendicular to the grooves thereof changes as the polarization direction in the measured light becomes different. It cannot be believed therefore that the optical spectrum value thus obtained is always correct at every wavelength .lambda..
In order to eliminate the above-mentioned drawback, there is provided an optical spectrum analyzer wherein the diffracted light is divided into parallel- and perpendicularly-polarized lights by a polarizing element and these parallel- and perpendicularly-polarized lights are detected independently of one other, as shown in FIG. 10.
Measured light (a) entering from outside is passed through inlet slit 1, processed to have parallel rays by collimator mirror 2, and introduced onto diffraction grating 3. The light diffracted by diffraction grating 3 is collected by camera mirror 4 which is a concave mirror, and it is applied to polarizing element 7 through outlet slit 5 and lens 6. This polarizing element 7 is made of calcite and serves to divide the light, which has been applied to it, into light (b) polarized parallel to the grooves of diffraction grating 13 and light (c) polarized perpendicular to the grooves thereof. These parallel- and perpendicularly-polarized lights (b) and (c) are received by light receiving means 8 and 9, respectively.
It is assumed in this optical spectrum analyzer that optical strengths obtained by the photoelectric conversion signals at an optional wavelength .lambda. applied from parallel- and perpendicularly-polarized light receiving devices 8 and 9 are I.sub.X (.lambda.) and I.sub.Y (.lambda.). It is also assumed that the wavelength sensitivity characteristics in both of the directions relative to diffraction grating 3 and including conversion efficiency are S(.lambda.) and P(.lambda.), as shown in FIG. 12. True optical strengths I.sub.S (.lambda.) and I.sub.p (.lambda.) of the light incident onto diffraction grating 13 in either of the directions can be thus obtained from the following equations (1) and (2) wherein loss caused by polarizing element 7 is included in wavelength sensitivity characteristics in either of the directions relative to diffraction grating 13: EQU I.sub.S (.lambda.)=I.sub.Y (.lambda.)/S(.lambda.) (1) EQU I.sub.P (.lambda.)=I.sub.X (.lambda.)/P(.lambda.) (2)
Therefore, an absolute spectrum value I(.lambda.) at this wavelength .lambda. which does not depend upon the polarization direction (angle .alpha.) in measured light (a) relative to grooves of diffraction grating 3 shown in FIG. 10, is calculated from the following equation (3): ##EQU1##
However, the following problem is still left unsolved by the optical spectrum analyzer which is arranged as shown in FIG. 10.
Polarizing element 7 which serves to divide the light, which has been diffracted by diffraction grating 3, into light polarized parallel to the grooves of diffraction grating 3 and light polarized perpendicular to the grooves thereof is usually made of calcite. However, optical the level is made low because of the scattering of light in calcite and because of the reflecting of light when it enters into calcite, and S/N ratio of parallel- and perpendicularly-polarized lights (b) and (c) applied from polarizing element 7 is also made low. High reliability is not obtained for the values of optical strengths I.sub.X (.lambda.) and I.sub.Y (.lambda.) which are obtained in both of the directions from optical photoelectric conversion signals applied from light receiving devices 8 and 9.
Reliability of the absolute spectrum value I(.lambda.) calculated from equation (3) using these measured optical strengths I.sub.X (.lambda.) and I.sub.Y (.lambda.) will be thus reduced.
Further, one light diffracted by diffraction grating 3 is divided into two polarized lights (b) and (c) by polarizing element 7. Absolute levels of divided polarized lights, (b) and (c) are thus made low. Therefore, the S/N ratio of each of these polarized lights (b) and (c) is further made low. As the result, the S/N ratio of each of the above-mentioned optical strengths I.sub.X (.lambda.) and I.sub.Y (.lambda.) is further made low and reliability of the absolute spectrum value I(.lambda.) finally calculated from equation (3) is thus reduced.
To summarize the above, the diffracting efficiency of the diffraction grating depends upon polarized waves in the case of the conventional optical spectrum analyzer of the diffraction grating type. In order to eliminate this polarized-waves dependency, there is used a technique of dividing light to be measured into component P (or parallel-polarized light) and component S (or perpendicularly-polarized light), detecting them and then multiplying them by the diffracting efficiency of the diffraction grating. However, level measuring accuracy is low because of loss caused by the polarizing element inserted.
The present invention is intended to eliminate the polarized-waves dependency of the diffraction grating while keeping the level measuring accuracy high.